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How can I change the sizes of my post featured thumbnail images?

You can change the sizes of any of the images used in various places throughout the site for post thumbnails by editing the functions/custom.php file. There is a function that dictates all the various sizes on lines 261 - 281. The function should look like this:

//add thumbnail support
if ( function_exists( 'add_theme_support' ) ) { // Added in 2.9
add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 160, 160, true ); // default post thumbnails
add_image_size( 'latest', 163, 109, true );
add_image_size( 'featured-full', 1000, 550, true );
add_image_size( 'featured-small', 650, 360, true );
add_image_size( 'footer-thumbnail', 40, 40, true );
add_image_size( 'spotlight', 300, 170, true );
add_image_size( 'spotlight-small', 230, 135, true );
add_image_size( 'loop-large', 630, 350, true );
add_image_size( 'loop-large-full', 960, 450, true );
add_image_size( 'panel-small', 190, 108, true );
add_image_size( 'trending', 105, 70, true );
add_image_size( 'widget-thumbnail', 70, 70, true );
add_image_size( 'single', 625 );
add_image_size( 'single-medium', 300 );
add_image_size( 'single-small', 180 );
add_image_size( 'single-full', 960 );
add_image_size( 'single-review', 350 );
add_image_size( 'related', 110, 70, true );
}

Find the line that corresponds to the image size you want to change. For instance, if you want to change the size of the images that appear in the spotlight slider, you would edit line 216. The two numbers on that line represent the width followed by the height in pixels. So, if you want to change the spotlight images from 170px height to 200px in height, you would change line 216 to this:

add_image_size( 'spotlight', 300, 200, true );

IMPORTANT - after changing any of these values, you must install and run the AJAX Thumbnail Rebuild plugin so your existing images can be resized to fit the new dimensions. It will not be necessary to run the plugin again after the first time (unless you change a value again).